Industrial communication and Protocol conversion


Thread Starter

Jayesh Trivedi


I have worked with Ethernet and serial communication devices. I referred various details on industrial networking and automation. When it is an industrial Ethernet, I understand it. However I get confused when it is related with other protocols like profinet, profibus, devicenet, EtherCAT etc etc. My basic questions are as below. Please help.

What is the basic purpose of building an industrial network. In a normal office / enterprise data networks, we build them to make the computers / servers communicate with each other to exchange the data. In the industry we do have only computers, but along with that many other devices like valves, motors, compressors, boilers, sensors etc. etc. Do we connect all them to each other to build an industrial network? and how?

(2) What topology is followed for industrial networks?

(3) If we connect various process control devices / valves / sensors / actuators etc. with each other and in turn with the control system / computers, how we are able to maintain the communication standards. As the different devices from different mfgr. may be able to communicate using one specific / typical protocol.

(4) Do we create different domain to define different segments to be created for building an industrial networks?

(5) What are the basic inputs / data at the bottom of the network. Like for certain process we are boiling a water in a tank up to 92 deg.C. Now we have installed a sensor which will generate corresponding signal for transferring it to the valve for necessary opening / closing. That would be routed through some controller which in turn will allow some electric / pneumatic power to finally actuate the valve to meet the process demand. Now this seems to be a close type of loop. How this is Physically as well as logically integrated with the control network. What signals / interfaces are used to connect them to the control system.

(6) What kind of wiring is used for connecting the devices.

(7) What is a fieldbus foundation, is it a wiring system or communication standard / protocol.

(8) How the PLC wiring is done from the field to the PLC?

(9) How we can select no. of analogue / digital IOs for PLCs being used.

(10) If we are using PLCs form different mfgrs., how we can connect them all to gather?

(11) Can we implement distributed network if the physical area to be covered is very large? What standards / rules we need to follow for wiring while installing a distributed network and finally drop them all to gather at central location?

Please help.
There is so much to be learned. Please start by reading some books on automation networking. You can start with my book Automation Networks published by ISA. You should also read A Consumers Guide to Fieldbus Network Equipment for Process Control, by Dick Caro available at or through ISA. If nothing else, just use Google to search for answers to your questions.

Dick Caro
(1) For digital communication in the plant environment there are three
categories of networks:
- Industrial Ethernets
- H2 fieldbuses
- H1 fieldbuses

Industrial Ethernets include: Modbus/TCP, PROFINET, EtherNet/IP, and many more

H2 fieldbuses include: Modbus/RTU, PROFIBUS-DP, DeviceNet, and many more

H1 fieldbuses include: PROFIBUS-PA, CompoNet, FOUNDATION fieldbus H1, ASI, and IO-link

Industrial Ethernets and H2 fieldbuses are both used at level 1-1/2
of the Purdue reference model. They connect similar devices such as
variable speed drives (VSD), motor starters, MCC, and wireless
gateways etc. These networks are fast but short distance, the devices
require separate power, and the network is not intrinsically safe.

There is somewhat of a trend that industrial Ethernet is taking the
place of "H2 fieldbus" (but NOT "H1 fieldbus"). For example:

- Modbus/TCP used in place of Modbus/RTU
- PROFINET used in place of PROFIBUS-DP
- EtherNet/IP used in place of DeviceNet

H1 fieldbuses are used at lowest level 1 of the Purdue model. They connect
sensors/transmitters and positioners/actuators/valves. These networks have
moderate speed but long distance, they provide power for the devices
eliminating the need for separate power, and some of the networks are
intrinsically safe ideal for hazardous areas

There is somewhat of a trend that H1 fieldbuses are taking the place of
hardwired 4-20 mA and on/off signals

As you can tell the application for the H1 fieldbuses are different from the
other two, and the characteristics are also very different from the other

If you are not familiar with the levels in the Purdue reference model, refer
to the illustration in this article:

(2) Ethernet uses star topology just like in the office; one cable to each
device from a LAN switch.

Fieldbus uses a multi-drop bus topology. A single cable connects several

For FOUNDATION fieldbus the cable usually runs from the DCS in the equipment
room out to a field junction box where it branches out to each device;
typically limited to 10-16 devices per bus.

(3) You can't fully network a plant using only a single protocol. As
mentioned in (1); for variable speed drives (VSD), motor starters, MCC use
an H2 fieldbus or an industrial Ethernet. I personally suggest PROFIBUS-DP
and PROFINET respectively because they support EDDL (

For field instruments like temperature, pressure, level, and flow
transmitters as well as analyzers, control valves, intelligent on/off
valves, electric actuators / motor operated valves (MOV) etc. use an H1
fieldbus. I personally suggest FOUNDATION fieldbus H1 because it has several
good characteristics not found in other H1 protocols and it also supports

(4) "Domain" isn't really used, but industrial Ethernet can certainly use
sub-networks; possibly organized by plant area. Each control system does it

Also for fieldbuses you "don't put all eggs in one basket". You may connect
maybe 10-16 devices per bus. Just like for 4-20 mA and on/off signals,
devices are first wired up to a nearby field junction box (this is called a
"spur" cable). 4-20 mA and on/off signals then used a multicore "homerun"
cable from the field junction box to the control system, but using fieldbus
a single pair runs from the field junction back to the control room (this is
called the "trunk" cable).

Note that a device like a valve positioner has 3 signals (1 valve setpoint
and 2 limit switches) which for hardwiring require 3 pairs of wire all the
way back to the control system and 3 I/O channels in the system. However,
with fieldbus all 3 device signals run over the same single pair all the way
which for the trunk cable and interface card is shared by many devices. This
dramatically reduces cable and I/O footprint. In the case of 6-12 signals
per MOV the savings are even more dramatic. On average devices may have 3
signals which mean for a bus with 10 devices you eliminate 30 pairs of wire
and 30 conventional I/O channels.

(5) To measure and control water temperature you still need a temperature
sensor. But instead of a temperature transmitter with a 4-20 mA output you
use a temperature transmitter with FOUNDATION fieldbus communication. The
value 92 degC or whatever is now communicated digitally as 92.00000 to the
controller where it becomes the PV in the PID algorithm; The PID output is
0-100% that is communicated digitally over the bus to the fieldbus valve
positioner which will throttle the valve accordingly. Both pneumatic and
electric actuators are available. Most things that used to use 4-20 mA or
on/off signals are now available with FOUNDATION fieldbus, and they are a
lot more intelligent. It is even possible to send the PV from the
transmitter direct to the valve positioner with the PID in the positioner.
This option is called "control in the field" (CIF) and does not pass the
loop through the controller. CIF is ideal for fast control loops.

Each device is identified by a different tag and each variable is identified
by name ensuring the information is logically separated even though they
share the same physical bus

(6) Industrial Ethernet uses Ethernet cable,

Each H1 fieldbus / H2 fieldbus use a different type of cable. These are also
widely available. For instance FOUNDATION fieldbus H1:

(7) The "Fieldbus Foundation" is the organization that develops and
maintains the "FOUNDATION fieldbus" protocol standard.

"Fieldbus Foundation" organization:

"FOUNDATION fieldbus H1" protocol:

(8) Field devices are individually wired up to a nearby field junction box
(this is called a "spur" cable). A shared single pair runs from the field
junction back to the control room (this is called the "trunk" cable). For
large installations 5-pair trunks are used, branching out to 5 field
junction boxes.

(9) With fieldbus you don't need to worry about a signal being analogue or
discrete, nor input or output, because all signals are virtual/soft. A pair
of fieldbus wires hooks up 10-16 devices, and it doesn't matter if each
device has only one signal or 16 signals either. You simply click in the
system configuration software selecting which signals in each device you
want to use. They call it "Virtual Marshalling" in the brochure I linked to
above. That is, in the fieldbus world you count DEVICES, you don't count
SIGNALS. If late in the design you discover you need to use some control or
feedback signal in the device you didn't originally plan for, just click in
the software to include it. No additional wiring, no additional I/O
channels. Similarly, if late in the design you discover a control valve
should instead be an on/off valve, just click in the software to make the
change. No need to change I/O card or marshalling wires. If an intelligent
on/off valve or control valve has to be changed to an MOV, again no problem
- just click in the software. No need to run more cable or install more I/O
cards. You can add more devices to the junction box (up to 16) without
laying additional cable.

(10) A PLC is connected up to a DCS or simple HMI software through
industrial Ethernet or an H2 fieldbus. Make sure all your PLCs use the same

(11) A large refinery or petrochemical complex has multiple areas. A common
design philosophy these days is to use several distributed "Field Auxiliary
Room" (FAR) sometimes called Satellite Rack Room (SSR) or Local Equipment
Room (LER) or Process Interface Building (PIB) etc. which is a building
housing part of the control system interfaces and controller for one or more
process areas. That is, a number of H1 fieldbuses from each process area
runs into the DCS interfaces and controllers in the FAR. The DCS controllers
are then networked together using a "control network" (level 2 in the Purdue
reference model - usually a proprietary protocol over Ethernet fiber optic
media) up to servers, operator workstations, and engineering work stations.
That is, in modern control systems, H1 fieldbus and Ethernet work together
complementing each other, just like USB and Ethernet on your office laptop.

To learn more about fieldbus and Ethernet take a look at the yellow book
"Fieldbuses for Process Control: Engineering, Operation, and Maintenance"
buy online:

I understand that the company for which you now work does not support
FOUNDATIONT Fieldbus HSE in their DeltaV product line, but the people
reading this list really should know about it. Thank you for your most
informative tutorial that was quite complete and accurate except for the omission of HSE.

FOUNDATIONT Fieldbus HSE is exactly an H2 level protocol developed because
the H2 protocols and other specifications of the ISA 50.02 standard were not
practical or cost effective. The Fieldbus Foundation began development of
HSE in 1999 and it was adopted as Type 5 of IEC61158 in 2001, one of the
original 8. The Fieldbus Foundation describes HSE at this website:

Obviously, HSE, as the name implies, is founded on the use of
commercial/industrial off-the-shelf high speed Ethernet. Since it is based
on unmodified Ethernet, it is free to use all variation of that technology
including fiber optic cable and any form of Category 5, 6, or 7 UTP
(Unshielded twisted pair) or shielded wiring, bridges, routers, and gateways
including Wi-Fi (although not yet tested by the Fieldbus Foundation.)

There are many applications of industrial Ethernet in H2 service in process
control systems; every DCS architecture, but they are mostly proprietary,
not open protocols. Two DCS suppliers do support FOUNDATIONT Fieldbus HSE in
this H2 service. The rest are proprietary.

I speak with users very often, and they feel that the use of open standard
networks is good for the industry and promotes competition, while
proprietary networks limit competition. I fully agree.

Dick Caro
CMC Associates
Certified Automation Professional (ISA)
Buy my books at the ISA Bookstore:
Wireless Networks for Industrial Automation (new 4th edition)
Automation Network Selection
Consumers Guide to Fieldbus Network Equipment for Process Control